This study investigated the role of steroids and gonadotrophin in final oocyte maturation (FOM) in the New Zealand snapper (Pagrus auratus) by means of in vitro techniques. Ovarian fragments containing vitellogenic oocytes were incubated for up to 48 h with a range of doses of gonadal steroids andlor human chorionic gonadotrophin (hCG). The ability of ovarian tissue to synthesize putative maturational steroids was assessed by measuring production of 17α,20β-dihydroxy-4-pregnen- 3-one (17,20βP) by ovarian fragments incubated with hCG and 17α-hydroxyprogesterone (17P). 17,20βP was consistently the most effective maturational steroid, inducing FOM at doses as low as 1 ng mI-1. The next most potent steroids were 20β-hydroxylated progestins and 17P, which were effective at relatively high doses (100 ng mI-1) only. The single androgen tested (testosterone) was least effective at inducing FOM. Oocytes were less responsive to hCG alone than to steroids, but the combination of hCG and steroids enhanced responsiveness in some incubations. Oocytes produced 17,20βP in response to treatment with 17P, and the response was augmented by co-treatment with hCG. HCG alone was ineffective at stimulating production of 17,20βP. The results of this study are consistent with reports that 17,20βP is generally the most potent steroid for the induction of FOM in teleosts.